In conventional labeling machines, mechanical elements are used to synchronize label feed, label cutoff and label application operations. One example of a mechanically synchronized labeling machine is disclosed in U.S. Pat. No. 4,108,710 to Hoffmann. The cost of mechanical timing gears and chains is a significant portion of the cost of labeling machines. The dedicated hardware elements of such systems makes it impractical to convert labeling machines from one style of article to another. To make a significant change in the size of labels applied or change the size of the cutter or vacuum drum, it is necessary to completely re-engineer the mechanical timing elements. Frequently, new gear drives or timing mechanisms must be developed to make such changes.
In an effort to improve on mechanically synchronized labeling machines, digitally controlled label feed rolls have been developed. An example of such an apparatus is disclosed in U.S. Pat. No. 4,552,608 to Hoffmann et al in which a digital data processing system senses the position of the cutter and controls a feed roller which is driven by a stepper motor. The rotational speed of the feed roller is controlled by the stepper motor. The machine operator provides inputs representative of the label length and location of registration marks with an optical sensor. While this approach provides some important advantages, the system disclosed is subject to several problems. First, stepper motors have limited precision and limited maximum speed which do not allow for instantaneous control of speed. Stepper motors are also generally fairly limited in horsepower and require complex and expensive power supplies. This approach also requires mechanical coupling between the cutter and the vacuum drum and a complicated gear system for timing the application of labels to containers.
A web registration control system disclosed in U.S. Pat. No. 4,361,260 to Hanlan discloses an electronic control for a web handling apparatus for cutting a web into segments. The web is driven by pinch, or nip, rollers having speed control based upon a registration mark which controls the location of the cut made by a cutter. The complex circuitry controls label feed and cut-off by means of a speed correction circuit. There is no disclosure of any system for varying the speed of the cutter for precise cut-off performance or for varying the speed of the cutter for smooth label segment transfer.
In general, label cutter mechanisms are designed to operate at a constant speed to cut a particular size of label. It has been discovered that label cut-off is optimized when the label speed and surface speed of the label cutter are matched during the cutting step. The surface speed of the vacuum drum is greater than the average speed of the label web because individual labels are spaced from each other on the vacuum drum. Ideally, label segments should move at the same speed as the vacuum drum at the time of label transfer. With conventional systems, system optimization is not possible because the optimum speed of the cutter during label transfer is different than the optimum speed for label cut-off.
Label cutter performance is affected by many factors including the type of label material to be cut. Label materials currently available include paper, polystyrene foam and various films including polystyrene, polypropylene and polyethylene. Foam and paper materials are easily handled under tension and are generally easier to handle than films which stretch. One of the most inexpensive types of films is polyethylene; however, it has not been considered useable for container labeling because of its tendency to stretch and distort under application of tension forces. While it is generally preferred to have a limited amount of tension on the label web during the cutting operation, the amount of tension that can be applied to a polyethylene web is extremely limited.
Once labels have been severed by the cutter, they are individually handled and supported by a vacuum drum. The vacuum drum in prior art labeling machines is preferably rotated at a higher rate of surface speed than the surface speed of the cutter. If not properly set, the vacuum drum can snap the label segment from the cutter, causing the label to shift. If the label shifts during transfer, it can result in misalignment of the label on the container.
These and other problems encountered by the prior art are addressed by the invention which is summarized below.